1. Field of the Invention
The present invention relates to a method for adjusting the gray scale in the reproduction of digitized images and particularly x-ray images in which it is desirable to visualize digital data which are representative of the image through a "window" having characteristics determined by the user. The invention is more specifically concerned with determination of the parameters of the window mentioned. The invention also relates to a system for processing digitized images and carrying out said method.
2. Description of the Prior Art
In digital radiology systems in which acquired images are transmitted and processed in digital form, pixel by pixel, the dynamic range of the acquisition means is appreciably greater than the capacity of the human eye to distinguish two adjacent gray levels. In other words, only a limited number of gray tones can be made to correspond to a large number of possible acquired digital values. Moreover, in the case of a given x-ray examination, only a fraction of the data contained in the digitized image is useful for examination purposes. By way of example, this fraction corresponds to a predetermined zone of absorption of x-rays by the structures to be observed. In this field of interest, it is a known practice to visualize the image by means of a "window". This method of display consists of making the entire scale of gray levels correspond to the useful range of digital values for the examination in progress. In other words, all of the values lower than or higher than this range are represented without distinction respectively by black or by white (or conversely) while the useful range of numerical values of the image is reproduced by all of the discernible shades (tones) of gray. The law of correspondence between the numerical values of the selected range and the shades of gray may be as desired but is usually linear. It will be apparent that, depending on the type of examination to be performed, the radiologist has to adjust the characteristics of the "window", or in other words to determine the limits of the range of numerical values which he desires to visualize. In the great majority of instances, these adjustments are performed by hand while the appearance of the image after "window processing" is continuously monitored on a television screen, for example. A usual practice consists, for example, in characterizing the window by two parameters which are referred to as a meanlevel parameter and a width parameter. The width of the window represents the number of numerical values which will actually be transcribed in shades of gray. This parameter is therefore representative of the contrast which will be greater as the width of the window is smaller. The mean level is given by the numerical value of the acquired image with which the mean shade of the gray scale is associated. This mean level is therefore chosen as a function of the nature of the structures to be examined. The user makes very frequent changes in these two parameters in order to subject the contents of the image to an exhaustive examination. Visual display installations are therefore equipped with means for controlling the variation of these two parameters. Control installations are provided on a front face with actuating elements such as push-buttons or rotatable knobs associated with pulse-generating circuits. Each generated pulse is processed by the electronic control system for the purpose of modifying the width or the mean level by a predetermined value. This elementary variation is difficult to determine. In fact, in the case of a substantial width, or in other words a low contrast, only a substantial variation in mean level is significant. On the other hand, in the case of a small width (high contrast), a minimum variation in mean level has consequences which are very clearly visible on the monitoring screen. In other words, in the case of a given choice of elementary variation in parameters which constitutes a compromise between the two extreme situations described above, the user may have the disagreeable impression of wasting his time by actuating the mean-level control knobs when the contrast is low since the resulting modifications of the image are insignificant. In the case of a high contrast, however, the same elementary variation in mean level will result in excessive modifications of the image. Thus there is a potential danger of preventing display of an intermediate configuration which might reveal clinically significant details. Up until the present time, some effort has been made to overcome this problem by providing several sets of keys for selecting a certain number of values of elementary variation but the resulting increase in the number of control elements also increases the risks of confusion and errors of operation since the user's attention is continually drawn to the monitor display screen and not to the control elements.
The present invention, however, makes it possible to solve the problem just described by reducing to a strict minimum the number of actuating elements required for adjusting the window.